Electrical amplifying system and method of operation



May 16, 1939. I J. JTNUMANSYI 1 2,158,248

ELEGTRIC AL AMPLIF'ZING SYSTEM AND METHOD OF OPERATION miu i Filed Sept. 11, 1934 3 Sheets-Sheet l PIC-3:7

Johannes Jacques A/umamr,

May 16, 1939- J. J. NUMANS ELECTRICAL AMPLIFYING SYSTEM AND METHbD OF OPERATION Filed Sept. 11, 1934 3 Sheets-Sheet 2- FIG-'4 I LL In venfor:

' May 16, 1939.

ELECTRICAL 'AMPLIFYING SYSTEM AND METHOD OF OPERATION J. J. NUMANS I 2,158,248

Filed Sept. 11, 1954 3 Sheets-:Sheet s om mrz afg //7 ven for;

Mafia/mes L/a cquea A L/ma/w, 5

Patented May 16, 1939 UNITED STATES ELECTRICAL AIWPLIFYING SYSTEM AND METHOD OF OPERATION Johannes Jacques Numans, The Hague, Netherlands Application September 11, 1934, Serial N 0. 743,610 In the Netherlands September 13, 1933 8 Claims.

The invention relates toamplifiers with thermionic tubes with three or more electrodes for the distortionless amplification of electric oscillations. Such amplifiers may be used in combination with loudspeakers for the reproduction of sounds (e. g. in the acoustical cinematography), or for the modulation of oscillations of a difierent frequency (e. g. in telephony transmitters) etc.

The invention relates more in particular to such amplifiers whereby a negative grid bias can be chosen in such a manner that the thermionic amplifier tube is adjusted approximately in the middle of its characteristic. However, the negative grid bias can also be chosen to such a value l5- -that the tube is adjusted at the bottom region of its characteristic; in this case preferably two tubes are used working in a push pull combination, in order to avoid serious distortion. The latter method is known from the British Patent The practice has learned that in such amplifiers very serious distortion occurs if an attempt is made to increase the output by increasing the amplitude of the signal voltage to such a degree 'that grid current is produced.

The invention aims to decrease this distortion considerably, respectively to nullify it altogether.

The invention will be explained by means of the drawings in which:

Fig. 1 shows a known amplifier circuit comprising means for applying a negative biassing potential to the control electrode of the amplifier.

Figs. 2, 3 and 4 show circuit arrangements according to the invention.

Figs. 5, 6 and 7 show graphs explaining the operation and advantages of the amplifier according to the invention.

It has been experienced that a particularly large distortion occurs with the flow of grid current, in the case that for the generation of the negative grid potential a so-called grid potentialsupply-apparatus is used, marked 5 in Figure 1. This grid potential-apparatus may consist of a rectifier with smoothing apparatus.

In Figure 1, I indicates an input transformer; 2 a, thermionic tube, 3 an output transformer; 4 a source of anode potential which is shunted by a condenser 6; 5 a source of grid potential of the kind already referred to above; 8 a. potentiometer for adjusting the required negative grid bias, and 1 a blocking condenser for blocking the negative grid potential and forming at the same time a bypass for the alternating potential of the signal.

Whenever in this circuit (which is a. well known one) the signal alternating potential is of such a magnitude, that grid current flows owing to the grid of tube 2 occasionally becoming positive and therefore attracting electrons, considerable distortion will result. i

The cause for this distortion .is to be found in the displacement of the operating point of the tube 2 on its characteristic as a result of an increase in negative grid bias.

This increase of the negative grid potential is 10 the result of the fiow of the grid current of the tube 2 through part of the resistance of potentiometer B, so that an additional potential is produced in it. The steady negative grid potential is thus increased and the result is that the oper- 15" ating point on the characteristic of tube 2 shifts to a region where distortionless amplification is no longer possible.

This phenomenon can be further explained with the aid of Figure 5. In this figure the grid current is plotted on the abscissa; this current occurs when the grid is made positive by the signal oscillations of Figure 1; on the ordinate is plotted the negative grid potential of tube 2 produced by the fiow of grid current.

The point to which the grid potential Vg1 is adjusted (at zero grid current) is at about 235.

The curve A indicates the increase in the negative grid potential Vgi of tube 2 when a grid current Igi flows; the figure clearly indicates that 3 this variation of grid potential is in fact considerable; under these circumstances serious distortion is found in practice to occur in this circuit.

tortion by reducing the resistance value of potentiometer 8 as much as possible; but by this procedure the energy loss in this resistance becomes very large indeed and it appears that a satisfactory result cannot be attained in this direction 40 potentiometer 8 with a device of which the volt- 45 a,

age-current-characteristic exhibits a thresholdvalue and of which the internal resistance is small, so that the value of the grid potential is practically exclusively governed by the magnitude of this threshold value and is practically inde- 5o pendent of the grid current it carries.

A suitable device for this purpose is a gas filled tube e. g. a so-called glow discharge tube, of which several may, if desired, be used in series. In Figure 1 this glow-discharge tube may 55 An attempt may be made minimize this dis 351,1;

be inserted in the circuit instead of condenser 1 or in parallel with it. By means of potentiometer 8 the potential is adjusted in such a way that a discharge occurs through the glow-discharge tube; it is important for the proper operation that this should occur. If grid current flows during the operation of tube 2, it will flow through the glow-discharge tube; this will produce only a very small change of potential across the glowdischarge tube becauseas is well knownthe potential across the glow-discharge tube is unafiected by the current between very large limits.

Experiment teaches that in this manner a very good result is obtained and that distortion does in fact disappear.

A drawback of the method which has just been suggested is that the potential across the glowdischarge tube has a fixed value which depends on the construction of the glow-discharge tube.

An accurate adjustment of the grid potential of tube 2 to difierent arbitrary values is impossible by means of such a device. In order to be able to adjust to a certain anode current, in this case, it is necessary to adjust the anode potential,

but this has many practical disadvantages.

According to the invention a thermionic tube with three or more electrodes can also be used as a threshold apparatus. A circuit arrangement for this purpose is shown by way of example in Figure 2. The auxiliary tube is herein marked 9; the grid potential of this tube can be adjusted by means of battery l0 and the anode voltage can be adjusted by means of the potentiometer 8. For a certain value of the grid potential of tube 9 a current will flow through this tube only when the anode voltage is above a certain value. Consequently the tube 9 may be employed as a threshold apparatus. The threshold value may be adjusted by adjustment of the grid potential of tube 9.

The operation of tube 9 in the circuit arrangement of Fig. 2 may be explained as follows.

When no signal voltage is being applied to the grid of tube 2, and consequently there is no grid current, the potential between the cathode and anode of tube 9 may be adjusted by means of the potentiometer 8 to such a value that the desired grid bias for tube 2 is obtained. Further the grid potential of tube 9 may be adjusted by means of battery It to such a value that a small current is flowing from the source of grid poapplied to the grid of tube 2 that grid current occurs in this tube, the potential between anode and cathode of tube 9 will show a tendency to increase because of the combined resistances of the tube 9 and the potentiometer 8. The tube '9, however, is then operated above its threshold value which means that its conductivity is rapidly increased. This increase of conductivity of the tube 9 causes the grid current of tube 2 to flow through tube 9 instead of mainly through the potentiometer 8, so that the potential across tube 9 remains substantially the same. Thus the tube 9 tends to keep the grid bias of tube 2 as constant as possible notwithstanding the grid current occurring in the tube 2.

The effect of the tube 9 may be further explained with the aid of Fig. 6. In this figure the current Iaz through the auxiliary tube 9 is plotted as abscissa and the potential Vaz between cathodeand anode of the auxiliary tube 9 is plotted as ordinate. This potential is, of course, equal to the grid potential Vgr of tube 2.

Curves a and b indicate two characteristics of tube 9, plotted at difierent values of the grid potential Vgz of tube 9. The threshold character can be readily seen; as this figure indicates, with the circuit of Figure 2, at a very high value of vaz Vgi, only a small current Iaz flows, but nevertheless the increase of V0.2 with an increase in M2 is only comparatively small.

Although the threshold value proper is at the value Ia2=0, it is suitable so to adjust the circuit that there is always a small current Iaz flowing, e. g. the current indicated at 3 which may correspond with point P1 on curve a.

The curve a is substantially flatter to the right right of P1 than to the left of P1 with the result that in the manner indicated, the value of Vaz can be kept constant to a better degree.

The result obtained with this circuit is indicated by curve B in Figure 5 which, as can be seen, is much flatter than curve A. It would have been possible to obtain a substantially better result if the tube 9 in Figure 2 had been a special tube having a low internal resistance.

A very much better result can be obtained if, according to a further feature of the invention, the grid potential of tube 9 is influenced by the potential between anode and cathode in such a way that with increasing potential the negative grid potential of tube 9 is decreased. As a result of such a decrease of the negative grid potential the passage of current through tube 9 increases as is necessary; the potential between anode and cathode of tube 9 is thus kept more constant.

It is, in fact, possible to attain the required result in several ways; a very simple and effectime method is indicated in Figure 3. The grid potential for tube 9 is here the drop of potential in resistance II.

The operation of the circuit of Fig. 3 may be explained as follows.

When no signal voltage is being applied to tube 2, and consequently no grid current flows, the adjustment and operation of tube 9 is the same as in Fig. 2, the only difference being that in Fig. 3 the grid potential of tube. 9 is supplied by the resistance H instead of by the battery I0. In Fig. 3 the current flowing from the source of grid potential 5 through the tube 9 and the resistance ll causes the grid of tube 9 to be negative with respect to the cathode. By making the resistance 1 I sufficiently large, the current through tub-e 9 can be made small.

If now a signal voltage of such value is applied to the tube 2 that grid current occurs, the potential between cathode and anode of tube 9 will show a tendency to increase because of the combined resistances of this tube and the series connection of the potentiometer 8 and the resistance l I. As a result hereof the current flowing through the tube 9 will be increased and the current delivered by 5 and flowing through resistance 1 I will be reduced, As the current through resistance I l decreases the negative grid potential of tube 9 also decreases and the conductivity of this tube increases, which results in a decrease of the potential-increase between cathode and anode of tube 9. Thus the negative grid bias of the tube 9 is controlled by part of the grid current of tube 2 flowing through the resistance II.

This effect may be further explained with the aid of Figure 6, by comparing the circuit of Fig. 3 with that of Fig.2.

- shift over curve a to point P2 so that Vaz and Vg2 (the grid potential of tube 2) change from 258 to 330.

If we consider Figure 3, however, the result is much better there. In Fig. the dotted line D (which relates to Fig. 3) indicates how the grid potential Vgz of tube 9, which is equal to the potential across the resistance H and which is plotted on the right hand abscissa, decreases when the current through tube 9, which is equal to the grid current 191 of tube 2, increases. When this efiect is applied to Fig. 6 it will be seen that on an increase of the current through tube 9 from 1:12:13 to Iaz=13, point P1 will shift to, say, point P3 on curve 12, instead of to point P2 of curve a. Consequently the grid potential of tube 2 will change only from Vg1=258 to Vg1=290, instead of from Vg1=258 to Vg1:330 as in the case of Fig. 2. The circuit of Fig. 3 is therefore better than that of Fig. 2.

The overall result obtained with the circuit of Figure 3 is depicted by curve C in Figure 5; it will be seen that in this case the grid potential is kept very nearly constant.

The great advantage of circuits according to the invention is especially apparent in the case of push-pull amplifiers which are adjusted by means of negative grid potential, to Work on the bottom region of the characteristic; under these circumstances a comparatively small increase of thenegative grid potential would result in very serious distortion. For this purpose it is desirable that both of the tubes working in pushpull should be capable of being adjusted individually to the required anode current because the tubes are seldom absolutely identical to one another.

In Fig. 4 a push-pull circuit according to the invention is shown. In this figure I indicates the input transformer supplying the signal voltage to the push-pull tubes 2. Negative grid bias is supplied to these tubes from the source of grid potential 5 by way of the potentiometer 8, the resistance M, the potentiometer l5 and the secondary windings of the input transformer I. I2 and I3 are two auxiliary tubes each serving for the same purpose as the auxiliary tube 9 in Fig. 2 and in Fig. 3. For each of the push-pull tubes of Fig. 4 the operation is the same as for the tube 2 of Fig. 3. The resistance l4 in Fig. 4 is for the purpose of producing a decrease of negative grid potential of the auxiliary tubes l2 and I3 when the current through these tubes is increased under the influence of grid current flowing through the push-pull tubes. By means of potentiometer 15 the grid potentials of the tubes 2 can be adjusted with respect to each other, While the grid potentials of the tubes 2 can be adjusted simultaneously by means of resistance [4.

If it is necessary to supply an auxiliary potential to the grids of the tubes 2 this can be accomplished in a simple manner by applying this auxiliary potential to the grid circuit of tubes l2 and I3, e. g. at the point marked E in Figure 4. Figure '7 indicates the change in grid potential (-dVm) of the tubes 2 under the influence of the applied potential E; apparently this function is practically linear which is a very desirable property for man purposes.

The advantage of the application of the auxiliary potential E in the manner suggested is due to the fact that in this case the source of potential 7 E hardly needs to deliver any current and is therefore unloaded; in many cases this is a considerable practical advantage. If the auxiliary potential E is connected directly into the path of the grid current of the tubes 2, the potential E would be afiected whenever a grid current flows because this grid current would have to flow through the source of potential E, thereby reacting on the source E.

This drawback is altogether overcome by the procedure described.

It will be clear that the negative grid potential of the tubes 2 can be regulated and adjusted to the desired value by adjusting the magnitude of the potential E. It is evident that this potential E can also be applied in the manner described to the circuits of Figure 2 and Figure 3.

I claim:

1. An electrical amplifier including. a thermionic amplifier tube having a. control electrode, a biasing circuit comprising a source of biasing potential adapted to apply a negative biasing potential to said control electrode, and means associated with the biasing circuit and adapted to prevent an undue increase of the negative biasing potential which might otherwise be produced on the occurrence of grid current flowing to the control electrode of the thermionic amplifier tube through the resistance of its source of biasing potential, said means comprising an auxiliary thermionic tube comprising a grid electrode and having its output circuit arranged in parallel to the source of biasing potential with its anode connected to the cathode of the thermionic tube, and auxiliary means for biasing the grid electrode of the auxiliary thermionic tube toimpart to said auxiliary tube an electric voltage-current characteristic presenting a threshold discharge voltage, and a low resistance.

2. An electrical amplifier including a thermionic amplifier tube having a control electrode, a biasing circuit adapted to apply a negative biasing potential to said control electrode, an auxiliary thermionic tube comprising a grid electrode and being associated with said biasing circuit, the anode of the auxiliary thermionic tube being connected to the cathode of the amplifier thermionic tube, and means for biasing the grid electrode of the auxiliary thermionic tube and causing the bias produced thereby to be controlled by grid current of the thermionic amplifier tube flowing through the biasing circuit of said amplifier tube.

3. An electrical amplifier including a thermionic amplifier tube having a control electrode, a biasing circuit adapted to apply a negative biasing potential to said control electrode, an auxiliary thermionic tube comprising a grid electrode and being associated with said biasing circuit, the anode of the auxiliary thermionic tube being connected to the cathode of the amplifier thermionic tube, and means for biasing the grid electrode of the auxiliary tube and causing the negative bias produced thereby to be reduced by grid current of the thermionic amplifier tube flowing through the biasing circuit of said amplifier tube, so as to produce an increased current conductivity of the auxiliary tube.

4. An electrical amplifier including a thermionic amplifier tube having a control electrode, a biasing circuit comprising a source of biasing potential adapted to apply a negative biasing potential to said control electrode, an auxiliary thermionic tube comprising a grid electrode and having its output circuit arranged in parallel to that section of the biasing circuit comprising the source of biasing potential, with the anode of the auxiliary thermionic tube connected to the cathode of the thermionic amplifier tube, and

biasing circuit comprising a source of biasing potential adapted to apply a negative biasing potential to said control electrode, an auxiliary thermionic tube comprising a grid electrode and having its output circuit arranged in parallel to that section of the biasing circuit comprising the source of biasing potential with the anode of the auxiliary thermionic tube connected to the cathode of the thermionic amplifier tub-e, and means for biasing the grid electrode of the auxiliary tube and causing the negative bias produced thereby to be reduced on the occurrence of grid current of the thermionic amplifier tube flowing through that section of the biasing circuit of said amplifier tube comprising the source of biasing potential.

6. An electrical amplifier including a thermionic amplifier tube having a control electrode, a biasing circuit comprising a source of biasing potential adapted to apply a negative biasing po- -tential to said control electrode, an auxiliary thermionic tube comprising a grid electrode and having its output circuit arranged in parallel to that section of the biasing circuit comprising the source of biasing potential with the anode of the auxiliary thermionic tube connected to the cathode of the thermionic amplifier tube, and means for biasing the grid electrode of the auxiliary thermionic tube and causing the bias produced thereby to be controlled by grid current of the thermionic amplifier tube flowing through that section of the biasing circuit of the amplifier tube that comprises the source of biasing potential, said means comprising a resistance connected between the cathode of the auxiliary thermionic tube and the source of biasing potential for the thermionic amplifier tube.

'7. An electrical amplifier including a thermionic amplifier tube having a control electrode, a

biasing circuit for said control electrode comprising a source of biasing potential, an auxiliary thermionic tube having its anode connected to the cathode of the thermionic amplifier tube and having its output circuit arranged in parallel to that section of the biasing circuit comprising the source of biasing potential, and a resistance provided between the cathode of the auxiliary thermionic tube and the source of biasing potential, a, point of said resistance being connected to the grid of the auxiliary thermionic tube.

8. An electrical amplifier including a thermionic amplifier tube having a control electrode, a biasing circuit adapted to apply a negative biasing potential to said control electrode, an auxiliary thermionic tube comprising a grid and be-- ing associated with said biasing circuit with its anode connected to the cathode of the thermionic amplifier tube, means for biasing the grid electrode of the auxiliary tube, and further means enabling the control electrode biasing potential of the thermionic amplifier tube to be varied, said further means comprising connections enabling a potential different from the biasing potential to be introduced into the grid circuit of the auxiliary thermionic tube.

JOHANNES JACQUES NUMANS. 

